CHEMISTRY COMMUNITY

Andres Reynoso 1J

Well... this is definitely what I needed to lighten up before the final lol

Andres Reynoso 1J

You need to add an inert electrode, such as platinum (Pt), when there are no other conducive metals in a half reaction in order to allow current to flow through the cell. You'd just need to add the electrode to the side that doesn't have a solid metal e.g. Zn(s) | Zn 2+ (aq) || Sn 4+ (aq),Sn 2+ (aq)...

Andres Reynoso 1J

Essentially, you would solve for the entropy at each individual step then add the totals up; this can be done because entropy is a state function. Your first step would to be to calculate the change in entropy due to the change in volume of helium gas. Step two would be to calculate the entropy chan...

Andres Reynoso 1J

I'm pretty sure Professor Lavelle said that we will need to know the functional groups we went over in lecture. We also talked about SN2 substitution reactions and went over those on the practice final, so I don't think it would hurt to review that as well.

Andres Reynoso 1J

No, $\Delta P$ does not have to equal zero in a bomb calorimeter, meaning pressure is not a constant.

Andres Reynoso 1J

Under what conditions does the rate law of the slow step determine the overall rate law of the reaction in a reaction mechanism? Is it when the slow step is the first step of the reaction mechanism?

Andres Reynoso 1J

Q: What is the name of 007's Eskimo cousin?
A: Polar Bond.

Andres Reynoso 1J

I think you're correct Michelle and agree with your conclusion; the value should be positive because of \Delta U=q+w . Additionally, part b should be work done on the system. Perhaps a typo in the question? Typo could make sense given the 982 kJ - 492 kJ = +490 kJ, not 491... I think you're correct,...

Andres Reynoso 1J

I'm pretty sure the catalyst was NO (g).

Andres Reynoso 1J

You can use the rate law rate=k[A]^{n} and plug in the units for each variable and simplify. For example, for zero order reactions: \frac{mol A}{L\cdot s} = k[A]^{0} \frac{mol A}{L\cdot s} = k For first order: \frac{mol A}{L\cdot s} = k[A]^{1} \frac{mol A}{L\cdot s} = k(\frac{mol A}{L}) \fra...

Andres Reynoso 1J

Q: Anyone know any jokes about sodium?
A: Na

Andres Reynoso 1J

Using the given half-life, we can use the formula $k=\frac{0.693}{t_{1/2}}$ to calculate the k value. From there, we can use the formula $ln[A]_{t}=ln[A]_{0}-kt$ to calculate time. [A]_t is the concentration remaining after time t.

Andres Reynoso 1J

In short, yes. The oxidation number represents the number of electrons that an element can lose (if negative) or gain (if positive), relative to the electronegativity of any other element is in combination with. Because O 2 and H 2 have the same electronegativity, respectively, their oxidation numbe...

Andres Reynoso 1J

No you do not always need to include Pt in your cell diagram. Platinum is used when there are no other inert metals in the reaction in order to allow current to flow through the cell. Most the time, you see Pt(s) being used when you are dealing with a cell that has a half-reaction that does not have...

Andres Reynoso 1J

I believe in lecture Friday, Professor Lavelle also used 'n' to represent the order of the reactant in the "rate law" ( $Rate=k\left [ R \right ]^{n}$ ) if that is what you are referring to when you say kinetics.

Andres Reynoso 1J

http://i1380.photobucket.com/albums/ah165/andresreynoso/chemcomm_zps6ilqogyd.jpg
Hopefully this helps!

*Edit: Photobucket doesn't allow third party hosting, so I just submitted the direct link.

Andres Reynoso 1J

Because the question asks for the standard Gibbs free energy of the second reaction, we assume that the temperature the reaction occurred at is 25°C since that is the accepted temperature for standard conditions.

Andres Reynoso 1J

The graph is essentially showing us the relationship between temperature and the (molar) Gibbs free energy of a substance at a given pressure and how they both correlate to what phase a certain substance in in. It shows us that at low temperatures, the Gibbs free energy is lowest for solids. Because...

Andres Reynoso 1J

Gibbs free energy is essentially a measure of how much energy we can get from a system. So if the free energy is at a minimum, then the reaction is at equilibrium and no more change in energy can be done in the system. If anyone would like to add, please feel free.

Andres Reynoso 1J

I'm pretty sure Cv and Cp are only to be used when the conditions dictate that the system is under constant volume or pressure, respectively, which is why it is often used when temperature is changing. For example, using Cp for a system where pressure is changing wouldn't work since pressure must be...

Andres Reynoso 1J

Professor Lavelle did an example on Wednesday where he asked at what temperature is Br(l)\rightarrow Br(g) spontaneous at 1 atm given \Delta H^{o}= 31.0 kJ/mol and \Delta S^{o}= 93.0 J/Kmol . He said that we were looking to find the temperature when \Delta G^{o}= 0 and said that neit...

Andres Reynoso 1J

The difference lies in what the question is asking. For 9.7 part a, the question asks for the entropy change at a constant pressure with a change in temperature. For this we use the known equation \Delta S=nC_{p}ln\frac{T_{2}}{T_{1}} where C p is the specific heat of a gas at constant pressure. For ...

Andres Reynoso 1J

Using bond enthalpies is the least accurate method to calculate standard reaction enthalpies because bond enthalpies are averages of the dissociation enthalpies of bonds from several different molecules. Essentially, they are the least accurate because it is an average, where other methods are more ...

Andres Reynoso 1J

Yes, when a system is isothermal it means that there is no temperature change in the system and thus the temperature would be constant. Only for ideal gases does this mean that the change in internal energy is 0 because, for ideal gases, \Delta U = \frac{3}{2}nR\Delta T and \Delta T = 0 (Anything mu...

Andres Reynoso 1J

Mercury in a thermometer is a closed system because only energy can be exchanged with its surroundings. Matter cannot be exchanged within the system. Here is a more detailed example for the thermometer's case: A thermometer is placed under someone's tongue. The heat (energy) from their body (surroun...

Andres Reynoso 1J

Yes it does stand for Kelvin. It is equivalent to degrees Celsius because specific heat equations usually involve temperature change . The magnitude for temperature change is the same for Kelvin and Celsius; so both units are interchangeable for specific heat capacity problems depending on which tem...

Andres Reynoso 1J

For temperature changes , it doesn't matter if it is degrees Celsius or degrees Kelvin because the value of the difference would be the same (e.g. 1 C heats to 5 C = +4 C; 274.15 K heats to 278.15 K = +4 K). When using a formula that doesn't involve temperature change, where the other units indicate...

Andres Reynoso 1J

What is the difference between these two terms? I do not think bond dissociation enthalpy has not been discussed in class but I came across the term reading online and am curious.

Andres Reynoso 1J

There is a negative sign in front of the value for heat (water) because it is the heat lost from the initial sample of water that is being gained by the water of the ice cube. The negative sign indicates the loss of that heat.

Edit: I agree with Jessica above.

Andres Reynoso 1J

I think that, for the scope of this class, the specific heat capacity of a substance will always be provided, given that the specific heat of a substance is determined through experimentation.

CHEMISTRY COMMUNITY

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Re: Lavelle: Professor by day, wizard by night

Re: Inert Electrodes

Re: Practice Midterm W18 #3A

Re: Organic Chem

Re: Cp & Cv?

Slow Step Determining Overall Rate

Re: Post All Chemistry Jokes Here

Re: Practice Problem

Re: Catalyst Example from lecture 3/12

Re: Rate Constant Units

Re: Post All Chemistry Jokes Here

Re: 15.27

Re: Oxidation state

Re: When do you need to include Pt? [ENDORSED]

Re: n

Re: Arithmetic in 11.19c

Re: 11.111

Re: Explain Figure 9.25...

Re: G at minimum

Re: Confused about work formulas and specific heats

Example from Wednesday's Lecture

Re: 9.7 and 9.11 formula differences

Re: Bondy Enthalpy Accuracy

Re: isothermal system

Re: HW #1 PART E

Re: Specific Heat Capacity

Re: Temperature [ENDORSED]

Bond Enthalpy vs. Bond Dissociation Enthalpy

Re: Question 8.41

Re: Heat Capacity